Hemolytic Complement (C) activity is based on the primary C pathway to induce hemolysis of erythrocytes sensitized with an optimal amount of anti-erythrocyte antibodies. Bactericidal C Activity involves the same components and mechanism as the hemolytic assay. In addition, bactericidal C activity also involves the alternative C pathway, i.e., alternative to C1, C4 and C2, but sharing the ability to activate the C system at the level of C3. Both hemolytic and bactericidal activities can be demonstrated by the radial diffusion method. It is hoped that, by using both tests in parallel, specimens with early C deficiencies (C1, C4, C2) late C deficiencies (C3, C5 through C9), or both can be screened. For example, absent or low hemolytic activity with normal bactericidal activity would suggest an early component defect.